The present invention relates to a microelectrical logging method and apparatus for carrying out measurements to differentiate open fractures from closed fractures and to determine the network of the open fractures in hard rocks pierced by a borehole. The method and the apparatus proposed are based on generating microelectric fields that penetrate the wall of the borehole. According to the invention, it is possible to determine the network of the fractures present in a hard rock formation, espeically to differentiate the open fractures constituting passageways connected into a system of communicating vessels showing remarkable hydraulic conductivity from the closed fractures that are permeable from outside of the borehole.
The apparatus and the method of the invention can be applied especially in the petrologic and hydrogeologic investigations and in prospecting sources of useful. minerals.
The determination of the network of the fractures penetrating a hard rock formation pierced by a borehole is a very important object of the geophysical investigations carried out through a borehold. In the hydrogeology, petrology and in the process of displaying the useful minerals the data received in this way are evaluated and processed under geophysical, geologic and mineralogic aspects. The importance of the measurements of such kind follows from the well known fact that the fractures which are present in a hard rock formation may form the path through which water enters mines and causes severe damage to the mines through flooding.
The background art consists of different solutions to the problem mentioned above; the most developed of them may be identified with the methods shown in the Letters Patent U.S.A. Pat. No. 4,468,623 and in the European early publication EP-A3-13 0 287 320. The essence of the methods cited above lies in application of a measuring downhole tool comprising pads made of electrically insulating material. and a system of metallic electrodes arranged within the pad. During the measurements, the downhole tool is lowered and pressed to the wall of the borehole in different places. The electrodes generate a micro-electric field in their narrow environment. The micro-electric field penetrates the rocks adjacent to the insulating pad and is distorted by them. The parameters of the microelectric field are measured and the data obtained thereby analyzed. The measured parameters are the electric current intensities and voltages and by processing them it is possible to determine whether fractures are present in the region investigated.
In the references cited, a method applicable to the measurements of sharp local inhomogeneities on the wall of a borehold in layers pierced by this borehold is proposed, wherein an electric field is generated by microelectrodes in a conductive way on the wall of the borehole. The microelectrodes are insulated from the drilling mud filling the borehole by insulating pads excluding direct electric contact to the conductive liquid present in the borehole. In the next step, the currents are determined at two or more localizations in the borehole by carrying out local measurements. The current intensities are coupled in a parallel way with one another in a current field generated perpendicularly to the wall of the borehole in the rock insulated from the drilling mud. The local inhomogeneities are determined on the basis of comparing the parallel current intensities: for each measurement the dimensionless ratio of the current intensities is computed and the maximum values of the dimensionless ratios are applied for displaying inhomogeneities on a major surface area divided into smaller regions.
The mentioned methods and apparatuses ensure highly effective measurements of the inhomogeneities on the wall of a borehole. However, they are characterized by two main disadvantages of generic character:
i. The investigations achieve penetration of the rocks only to a shallow depth, i.e., in the practice it is impossible to differentiate the open fractures forming parts of a communicating fracture network showing remarkable hydraulic flow capabilities from the closed fractures produced mainly by the boring operation due to altering the stress distribution system in the interior of the rock. This impossibility follows from the fact that the networks of both types of fracture are deeper than the radial investigation depth of the microelectric field in the rock formation.
ii. The background art is based on the obviously inaccurate assumption that the fractures constitute generally straight line formations in the wall of the borehole. Hence, during the data processing the fractures are supposed to traverse the rock along straight lines. This assumption results in erroneous determinations of the trend directions of the fractures - the fractures traversing the rocks along broken and curved lines cannot exactly be determined.